What happens if one person dies while the spell "Exchange of the two minds" is active, do both die, or just one?
This is not just a hypothetical question, it has implications. A magus could use the spell (or one of shorter duration) to hijack someone elses body before doing something that has a high probability of ending in death.
Imagine that a magus must face down a dragon he has little hope of surviving against. He is unable to send someone else and is unwilling to sacrifice his own life. So the magus swaps into the body of his apprentice (or someone else, as long as the magi can work magic in the new body), sets out to do the task and dies. But who died?
The situation might be similar to the one that happens if one of the persons involved in the spell fails his Stamina roll of 3+ and dies during its casting. The spell doesn't mention what happens to the one who survives, if he even survives. So I am just assuming that he does and I also assume that the survivor gets his mind back as well, no harm done.
So what happens then if one person dies while the spell is active? If the spell was momentary I would say that the mind transfer was permanent, but its not (for game balance reasons?). Instead it has a specific duration which means that the minds are returned when the spell ends. Does the spell end prematurly when one of them dies, can a spell still be active even if it has nothing to act on?
If one look at the nature of the spell I can see two things of interest. First of all it's a Rego spell, which means it can move somethings location but not change what it is. The second thing is the Limit of the Divine and the lesser Limit of the Soul, my judgement is that this means that the mind is still very much connected to its original soul even after being moved (but I could be wrong). This would mean that the spell is not considered a Copy Paste spell on minds and it is not possible for a relocated mind to commit sins and make the other soul go to Hell for it.
Based on this, my assumption would be that if one person dies, it would be that it's the original owner of the body who dies, since his soul resided inside that body. When this happens, the spell ends (this is a guess, but based on that a spell needs its original targets to remain active) and the survivor gets his own mind back.
But another possibility would be that the relocated mind dies as well, although I am not certain why this would be the case since a mind is immaterial (not sure if severe brain trauma should be an exception), but if the relocated mind died as well it would result in one properly dead person and one mindless husk.
My own judgement say that, unless a death punishes both people targetted by the spell, this is much too inviting to abuse and should be ruled against. All it takes to be safe while swapping into a new expendable body would be a non-ritual version of the spell, a stamina score of 3 and a Spell Mastery score of 1 to get rid of the remaining botch die (victim not included, misuse of spell invalidates warranty, only for magi age sixtysix and up).
It would be nice to see what others has to say about this.